This invention relates to an argon gas laser device and more particularly to the mirror construction of an optical resonator thereof.
Recently, there has been wide use of laser devices in the fields of photography, photocopying and the like. The laser is used for scanning a photosensitive plate or luminescent screen for reproducing an image. Generally, a photosensitive plate or the like has maximum sensitivity in the short wavelength region of the light spectrum; consequently, it is desired to use laser devices emitting light of short wavelength, such as emitting light between the green and ultraviolet region.
It is well known that an argon gas laser is the most suitable laser device for this purpose. Laser wavelengths emitted by this device are 4545 .ANG., 4579 .ANG., 4658 .ANG., 4727 .ANG., 4765 .ANG., 4880 .ANG., 4965 .ANG., 5017 .ANG., 5145 .ANG. and 5287 .ANG.; the emitted wavelengths of 4765 .ANG., 4880 .ANG., 4965 .ANG., 5145 .ANG. provide a high laser gain, while the remaining wavelengths provide a much lower gain. It is desirable to have monochromatic oscillation of one of these higher gain wavelengths to effectively emit a strong laser light.
There are several prior art systems to selectively emit monochromatic laser light of a predetermined wavelength. One such laser device utilizes an optical resonator wherein a prism is provided to disperse the undesired wavelengths. In that system, however, there is a tendency for the resonator's oscillation to decrease due to scattering losses on the surface of the prism and transmission losses within the prism. These losses occur even if the prism is disposed at the Brewster angle. In this prior art system, adjustment of the optical resonator's prism is very complicated and precise, and continuous and stable operation of the laser is very difficult to maintain. This is due to the fact that small changes in operating conditions, such as ambient temperature or mechanical vibration, sharply affects the performance of the laser since the prisms dispersion characteristic or alignment can concomitantly vary.
The use of a diffraction grating has also been proposed in the prior art. Such a system, however, has substantially the same disadvantages described above for the prism type optical resonators.
A further system has been proposed whereby filtering properties are added to a mirror of an optical resonator to produce the desired monochromatic output of the argon laser (see Japanese Patent Publication No. 44-29436). In this prior art system, the mirror comprises multiple dielectric layers, and the spectral dispersion properties of the multiple layers are utilized to filter certain laser wavelengths. As described above, the light emitted from an argon gas laser produces ten wavelengths wherein the difference between the longest wavelength and the shortest wavelength is 742 .ANG. and the interval between two adjacent wavelengths are within the range of 34 .ANG. to 142 .ANG.. The spectral characteristics of the combined multilayer mirror is practically determined from refractive indices of the combined dielectrics, and has a relatively wide reflectance band (see Japanese Publication No. 44-29436: B-FIG. 3). For example, in the case of the dielectric combination of ZnS and MgF.sub.2, the reflectance band is approximately 1800 .ANG..
Accordingly, in this prior art system, a high reflectance mirror and an output coupling mirror are used to produce an oscillated output composed of virtually only the 5145 .ANG. wavelength. The construction of the multilayered high reflective mirror is designed to have a lower end cutoff of the reflectance band between 5017 .ANG. and 5145 .ANG.. As a result, the 5017 .ANG. wavelength and the other shorter wavelengths are cut off, while the two remaining longer argon laser wavelengths, 5145 .ANG. and 5287 .ANG., are reflected. However, since the 5287 .ANG. wavelength has a negligibly small oscillated gain or output, the system produces an output substantially comprising the high gain 5145 .ANG. wavelength.
With this prior art system, the high reflective mirror can also be designed so that the oscillated output is composed of virtually only the 4765 .ANG. wavelength. In this case, the upper end cutoff of the reflectance band is selected to be between 4765 .ANG. and 4880 .ANG.. As a result, the 4765 .ANG. wavelength and the other shorter wavelengths are reflected while the 4880 .ANG. and the other longer wavelengths are cut off. Since, as discussed above, the 4545 .ANG.-4727 .ANG. have a negligibly small gain, the laser produces an output substantially comprising the high gain 4765 .ANG. wavelength.
This prior art system, however, has the disadvantage of permitting the monochromatic selection of only the uppermost and lowermost high gain wavelengths, 4765 .ANG. and 5145 .ANG., respectively. The other intermediate high gain wavelengths (i.e., 4880 .ANG. and 4965 .ANG.) cannot be obtained. In fact, the highest gain wavelength cannot be monochromatically obtained (i.e., 4880 .ANG.). In addition, since this prior art method requires the cutoff wavelength to be sufficiently close to either the uppermost or lowermost high gain wavelengths, any change in the reflectance band can produce instability of the system. There is a tendency for the spectral characteristics of a multilayer mirror, comprising, for example, ZnS and MgF.sub.2, to frequently change which, in turn, can affect the oscillation and stability of the laser. It has been found that the spectral characteristic changes due to a tendency for the thickness of the multilayer mirror to shrink.